Well all I know is that they don't have the rights to Superman, Batman, Wonder Woman, Green Lantern yet which are major copyrighted characters owned by the WB, therefore, they need to have permission.
Flash is too. Still, if they got the Flash, then they should be able to use Aquaman, Martian Manhunter, Blue Beatle and Booster Gold. The more, the merrier. I mean look, DC's Legends of Tomorrow made up their own characters to add more heroes. That's not enough to make a Justice League. Not to mention they added two villians, Cold and Heat Wave...they're not heroes.
I wish they wouldn't have done this. Still on the other hand, if they were going to make a Justice League TV show with the trinity in it, then I think it would ruin the upcoming Justice League movie itself. The whole idea, the excitement of seeing them altogether on screen for the first time is huge. Seeing that first in a tv series would make the film less exciting.
Still there is a possibility Supergirl will crossover with Arrow and Flash, by that point, it will mean that Superman exists in the Arrow/Flash-verse.
I think it might be interesting, but I'm only saying that maybe it will, or maybe not....if after the JL movies or by the time the DC cinematic universe has been established and are in the middle of their stage, then the DC television universe can make their own Justice League series with Superman, Batman, Wonder Woman, Grant Gutsin's Flash, Stephen Amell's Arrow, etc...except how will they have a budget for that?
I'd feel like there would be a good balance though. Supergirl is innocent as Flash is, and Supergirl being part of the team would make it light. Arrow would be serious, then you have of course the trinity, all pretty serious and straight personalities.